In sound recording and reproduction, equalization is the process commonly used to alter the frequency response of an audio system using linear filters. Most hi-fi equipment uses relatively simple filters to make bass and treble adjustments. Graphic and parametric equalizers have much more flexibility in tailoring the frequency content of an audio signal. An equalizer is the circuit or equipment used to achieve equalization. Equalizers adjust the amplitude of audio signals at particular frequencies.
Equalizers are used in recording studios, broadcast studios, and live sound reinforcement to correct the response of microphones, instrument pick-ups, loudspeakers, and hall acoustics. Equalization may also be used to eliminate unwanted sounds, make certain instruments or voices more prominent, enhance particular aspects of an instrument's tone, or combat feedback in a public address system. Equalizers are also used in music production to adjust the timbre of individual instruments by adjusting their frequency content and to fit individual instruments within the overall frequency spectrum of the mix.
An equalizer receives an input audio signal X, applies a transfer function H on it, and outputs an audio signal Y. This process can be represented (in the frequency domain) as                Y=H X.        
The linear filter used by the equalizer is designed to implement the transfer function H. However, when the equalizer switches from one transfer function to another transfer function, the transition can be abrupt and aesthetically unpleasant to listeners of the audio.